Manhole cover
A maintenance cover or manhole cover is a removable plate forming the lid over the opening of a maintenance hole, to prevent someone from falling in and to keep unauthorized persons out.
Manhole covers usually weigh more than 100 pounds (50 kg), partly because the weight keeps them in place when traffic passes over them, and partly because they are often made out of cast iron, sometimes with infills of concrete. This makes them inexpensive and strong but heavy. They usually feature "pick holes", in which a hook handle is inserted to lift them. Specialized manhole hooks are manufactured for this purpose.
India has become the world leader in manufacturing manhole covers, driving many manufacturers in other countries out of business. Its extremely low labor costs have overcome even the cost of shipping such heavy objects.
Although the covers are too large to be collectible, their ubiquity and the many patterns and descriptions printed on them has led some people to collect pictures of covers from around the world.
In some areas the term "manhole" is now regarded as a sexist term, especially now that women are increasingly working in the utility sector. The term "maintenance cover" is used by some cities.[citation needed]
Despite their weight and cumbersome nature, manhole covers are sometimes stolen, usually for resale as scrap, particularly when metal prices rise.
Why are they round?
The question of why manhole covers are typically round, at least in the U.S., was made notorious by Microsoft, who began asking it as a job-interview question. Originally meant as a psychological assessment of how one approaches a question with more than one "right" answer, the problem exerts a strange fascination on people, who seek to find alternate explanations, from the pragmatic and satirical to the philosophical. One practical answer being, "Manhole covers are round because manholes are round."
Reasons for the shape include: A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it was inserted diagonally in the aperture (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.) Round tubes are the strongest and most material-efficient shape against the compression of the earth around them. The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using horizontal boring mills. Circular covers can also be moved around by rolling, and they need not be aligned to put them back.
However, other manhole shapes can be found, usually squares or rectangles. Nashua, New Hampshire may be unique in the U.S. for having triangular manhole covers that point in the direction of the underlying flow. The city is phasing out the triangles, which were made by a local foundry, because they are not large enough to meet modern safety standards, and larger triangles cannot be found.
Trivia
Can a race car lift a manhole cover?
Formula 1 race cars create so much vacuum due to their aerodynamics that they can lift a manhole cover above the ground. During races on city streets, the manhole covers must be welded down to prevent injury. In 1990, while racing in Montreal, racer Jesus Pareja's car struck a manhole cover that was lifted by the ground effect of the car he was following, causing his car to catch fire.
The first man-made object in space?
An urban legend claims that a manhole cover was accidentally launched from its shaft during an underground nuclear test in the 1950s, at great enough speed to leave the solar system. The story is based on a real incident in August 27 1957 during the Operation Plumbbob nuclear tests, where a heavy steel plate cap was blasted off the test shaft at tremendous velocity, never to be seen again. However, it is doubtful that it left the atmosphere.
See also
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References
Printed references:
- Melnick, Mimi (1994). Manhole Covers, MIT Press, USA. ISBN 0-26213-302-4
- Stuart, Diana (2003). Designs Underfoot: The Art of Manhole Covers in New York City, The Lyons Press, USA. ISBN 1-58574-639-8
External links
- Rampant manhole theft in China, The Age, Feb 8, 2006
- Manhole Covers in Space - exhaustive article by Debbie Moorhouse
- The Pascal-B nuclear test